Modern meat grinding systems not only grind meat, but also separate meat from undesirable material, such as gristle, hard fat, connective tissue, sinew, and bone. Such systems typically provide the unprocessed meat to a pump or screw system, which delivers the unprocessed meat to a knife rotating against a perforated plate. The knife cuts the meat into smaller pieces as the pump or screw forces the meat through the perforated plate. Centrifugal force generated by the pump or rotating screw drives the meat toward the edges of the plate and concentrates the undesirable material near the center of the plate. The plate typically contains an opening into a pipe that directs the undesirable material to a storage container. Once enough of the undesirable material has been collected, the undesirable material is fed through a second meat grinding system to further separate any remaining meat from the undesirable material in a reclamation process similar to that described above. The meat reclaimed from the undesirable material is then added back to the meat ground in the initial pass and the further resulting undesirable material is discarded or otherwise processed separately.
Prior art meat grinding systems use a feed screw to move material through the system and assist in separating the meat from the undesirable material. Such systems often use a feed screw having a large pitch at the meat hopper inlet, with the pitch of the feed screw reducing to a small pitch flight as it approaches the grinding plate. Reducing the pitch of the feed screw reduces the material handling volume output of the feed screw, increases the velocity of material being moved by the screw, and increases the hydrodynamic pressure applied to the material. The rise in pressure and velocity is proportional to the change in pitch of the feed screw. Near the grinding plate, the pitch of the feed screw increases again, reducing the pressure and velocity of the material.
One drawback associated with such prior art feed screws is that they do not increase the velocity and pressure of the material enough to sufficiently to dislodge enough undesirable material, such as gristle and bone, from the meat. It would be desirable to provide a feed screw that better controlled flow of the material and separation of the undesirable material from the meat.
Another drawback associated with prior art feed screws is that while they change in pitch, to increase the velocity and pressure of the material to dislodge undesirable material from the meat, this increase in velocity and pressure hinders the centrifugal precipitation of the undesirable material toward the axis of the feed screw. Such centrifugal precipitation of the undesirable material toward the axis of the feed screw is desirable, as it facilitates removal of the undesirable material from the meat. It would therefore be desirable to increase the velocity and pressure of the material to dislodge undesirable material from the meat, but then decrease the velocity and pressure of the material, to facilitate centrifugal precipitation of the undesirable material toward the axis of the feed screw for reclamation.
The difficulties encountered discussed here and above are substantially eliminated by the present invention.